1. Field of the Invention
This invention relates to a developing apparatus for visualizing a latent image on a latent image support in a dual component developer on an electrophotographic copier, an electrophotographic printer, etc., and in particular to an improvement in a developing apparatus of the type wherein a developing bias with an AC component superimposed on a DC component is applied to a developer support.
2. Description of the Related Art
Hitherto, various types of developing apparatuses used with electrophotographic copiers, electrophotographic printers, etc., have been provided; for example, a developing apparatus of the type wherein a developing bias with an AC component superimposed on a DC component is applied between a developer support and a latent image support by using a dual component developer is already known.
In this type, the edge effect (electric force line diffraction from a latent image edge section on the latent image support to a non-image section) can be suppressed by a strong development electric field of the AC component and moreover the developer mobility between the latent image support and the developer support can be improved, so that an electrostatic latent image can be reproduced more faithfully and image quality (reproducibility of a solid part, thin line reproducibility, etc.,) can be improved.
Such a developing apparatus generally adopts a technique of sensing the toner concentration and replenishing with toner if the toner concentration falls below a predetermined level from the viewpoint of providing development.
A conventional development apparatus is already known wherein degradation of a developer is considered and a given print concentration is provided by changing a DC component of a developing bias (for example, refer to Japanese Patent Laid-Open No. Sho 59-38765).
However, if toner concentration control as described above is performed, fluidity of toner changes with time and when the toner fluidity is degraded, a partial transfer failure occurs; a new technical problem is found.
That is, if a developing bias with an AC component superimposed on a DC component is applied to a developer support, a development electric field between the developer support and a latent image support is formed, and toner on the latent image support is close to a closest packing state and is therefore hard to transfer as compared with the situation wherein a developing bias of only AC component is used.
If a developer is used over time under such circumstances, for example, a coating layer of carrier coated with a resin is peeled off, thus the resistance value lowers, resulting in a strong development electric field. On the other hand, the external additive surrounding toner may be peeled off or embedded. Thus, the contact area between toners increases and in addition, toner is placed near the closest packing state under the strong development electric field.
Therefore, if the developer is used with time, the toner fluidity inevitably is degraded, the adhesion force between toners on the latent image support increases, and it becomes difficult to completely transfer the toners onto paper.
A mono component developing apparatus may be used for controlling the effective value of a developing bias (AC component) in response to the remaining amount of a developer in the developing apparatus (for example, refer to Japanese Patent Laid-Open No. Sho 59-184375). This apparatus is provided to solve the problem proper to the mono component developing apparatus, namely, change of development with time associated with the particle diameter of the developer (toner) in the developing apparatus (based on preferential adhesion of toner of small particle diameter to a developer support); it cannot be adopted immediately as technical means for solving the above-described technical problem of the dual component developing apparatus.